1. Field of the Invention
This invention pertains, in general, to making analog form playbacks from digitally recorded data (e.g. seismic data) which has been digitized from wide dynamic amplitude range analog form signals initially generated by transducers, such as geophones, in response to acoustically induced seismic disturbances; in particular, to the making of analog form playbacks such as oscillograms which are approximate but very useful reproductions in compressed range of the wide dynamic range amplitude-versus-time characteristic curves of the analog signal initially generated by the aforementioned transducers.
The aforementioned oscillograms may be made substantially simultaneously with the acquisition of the signals generated by the geophones; i.e., the system may function as a monitoring system. In the alternative, the oscillograms may be made at any convenient time after the acquisition of the signals generated by the geophones; i.e., the system may function as a playback system.
2. Brief Description of the Prior Art
In seismic exploration work each acoustically driven geophone generates wide dynamic amplitude range signals in analog form. When such signals are processed through a digital seismic recording system there is produced a high fidelity record in digital form covering the wide dynamic range of amplitudes of the seismic signals. The reason that the digital form record is referred to herein as a high fidelity record is because the signal amplitudes are recorded accurately throughout their wide dynamic range; e.g. many binary digit, or bit positions are used to precisely record the highest signal amplitude as well as the lowest where the range (i.e., the ratio of the highest signal amplitude to the lowest signal amplitude) may be of the order 10.sup.6.
The subject invention provides a method and apparatus for making analog form oscillograms from the recorded digital data. The oscillograms are of relatively lower fidelity than the aforementioned digitally recorded data. Although the oscillograms are of relatively lower fidelity serious distortions are, nevertheless, not introduced in reconverting the digital data to analog data for the purpose of making compressed amplitude range oscillograms.
The techniques to record, in digital form, wide dynamic amplitude range analog signals initially generated by geophones are disclosed in, among others, the following: U.S. Pat. No. 3,241,100 issued Mar. 15, 1966 in behalf of R. J. Loofbourrow and entitled "Digital Seismic Recording System"; U.S. Pat. No. 3,264,574 issued Aug. 2, 1966 on behalf of R. J. Loofbourrow and entitled "Amplifier System"; U.S. Pat. No. 3,603,972, which issued Sept. 7, 1971 to James R. Vanderford and is entitled "Amplifier System". The hitherto known techniques to make analog displays of such digitally recorded data are disclosed in, among others, the following: U.S. Pat. No. 3,685,046, which issued on Aug. 15, 1972 to Donald P. Howlett and is entitled "Seismic Playback/Monitor-System" and in U.S. Pat. No. 3,872,465, which issued on Mar. 18, 1975 to Robert J. Loofbourrow and is entitled "Seismic Playback/Monitor System".
In the aforementioned Patents of Vanderford and Howlett there is described a system wherein portions of an analog signal are converted to digital words wherein each digital word occupies a number of binary digit, or bit, positions. Moreover, each such digital word is recorded in floating point form. The floating point form of notation allows greater flexibility of operation and easier handling of numbers differing greatly in magnitude from each other. (See, for example, the textbook "Digital Computer Primer" by E. M. McCormick, 1959, published by McGraw-Hill Book Company, Inc., beginning at page 152.) In the system disclosed in the Vanderford and Howlett patent applications, hereinbefore identified, a floating point digital number, or word, in the form of a mantissa or argument, and an exponent is recorded on a suitable storage medium such as magnetic tape. The floating point digital word represents the instantaneous absolute seismic voltage amplitude as it is introduced to a floating point amplifier system. The dynamic range of the floating point word may be in excess of 200 dB, if necessary, to cover the dynamic range of input signal (equivalent to a digital number sonsisting of 36 binary digits, or bits).
As a specific example the floating point word as set forth in conventional algebraic form is as follows: EQU e.sub.in =.+-.AB.sup.-E (equation 1)
wherein e.sub.in represents the absolute magnitude or amplitude of the floating point word; A represents the mantissa, or argument, portion of the word; B represents the base, or radix, of the number system used (B=10 in the decimal, or base 10, system or B=8 in the octal system); and E represents the exponent.
As is suggested in the patents hereinbefore identified, the floating point digital word is in the form EQU Q=.+-.AB.sup.-E (equation 2)
wherein Q represents the absolute magnitude of the amplitude of the input signal delivered to an arrangement of amplifiers, each of which has a gain of eight (8) and hence the base B in equation 1 becomes 8 in equation 2; the mantissa A represents the output amplitude of a particular amplifier in the aforesaid arrangement; and E, the exponent, respresents the number of amplifier stages of gain of 8 through which the aforesaid input signal has been processed.
In order to record the floating point digital word of equation 2 in a binary register with, for example, 144 dB of dynamic range and with 14 binary digit, or bit, accuracy, 18 bit positions would be required where the mantissa A is represented in binary form (i.e., where the base, or radix, of such a number systemm is 2) and where the exponent E is also represented in binary form. Of the 18 bits required: one bit represents the sign allowing for bipolar input-output capabilities; 14 bits represent the mantissa A; and 3 bits represent the exponent E.
Although there are many advantages to recording seismic signals in digital form, there still remains the need to make available to a seismic prospector a visible display or recording of the seismic data, or portions thereof. Conventionally, the visible record is an oscillogram. Often, it is desirable for a seismic prospector in a seismic field crew in a location remote from a main data processing center to take a quick look at a portion of the seismic data from time to time. For example, a seismic prospector may wish to make some interpretations with respect to the oscillograms in order to coordinate such data with geological data.
It is an object of the present invention to overcome the difficulties and limitations recited above.
Accordingly one object of the invention is to convert data from digital form to analog form.
Another object of the invention is to provide new and useful methodology for converting data from digital form to analog form.
Another object of the invention is to provide new and useful apparatus for converting data from digital form to analog form.
Another object of the invention is to convert wide dynamic amplitude range digital data (e.g., seismic data) to analog form displays such as oscillograms, or wiggle traces.
Another object of the invention is to convert wide dynamic amplitude range digital data to analog form data as oscillograms which oscillograms are selectively compressed reproductions of wide dynamic amplitude range analog signals which existed prior to their conversion to said digital data.
Another object of the invention is to convert wide dynamic amplitude range digital form data to analog form data having selectively compressed amplitude without introducing serious distortions.
Another object of the present invention is to convert wide dynamic amplitude range digital form data where the average amplitude increases or decreases in arbitrary fashion that is not known beforehand.
The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood however that the drawings are for illustrative purposes only and are not to be construed as defining the limits of the invention.
Although the invention is hereinafter described as being employed in conjunction with digital seismic recording systems such as those disclosed in the patents hereabove identified it is, nevertheless, to be understood that the invention's field of use is not processing.